The field of the invention is x-ray and gamma-ray cameras based on the Compton-scatter principle.
Scintillation cameras are widely used as diagnostic tools for analyzing the distribution of a radiation-emitting substance in an object under study, such as for the medical diagnosis of a human body organ. A typical scintillation camera of the Anger-type is described in U.S. Pat. No. 3,011,057.
The Anger-type scintillation camera can take a "picture" of the distribution of radioactivity throughout an object under investigation, such as an organ of the human body which has taken up a diagnostic quantity of a radioactive isotope. A scintillation camera of the Anger-type produces a picture of the radioactivity distribution by detecting individual gamma rays emitted from the distributed radioactivity in the object that pass through a collimator to produce a scintillation in a thin planar scintillation crystal. The scintillation is detected by a bank of individual photomultiplier tubes which view overlapping areas of the crystal Appropriate electronic circuits translate the outputs of the individual photomultiplier tubes into X and Y positional coordinate signals and a Z signal which indicates generally the energy of the scintillation event and is used to determine whether the event falls within a preselected energy window. A picture of the radioactivity distribution in the object may be obtained by coupling the X and Y signals which fall within the preselected energy window to a display, such as a cathode ray oscilloscope which displays the individual scintillation events as spots positioned in accordance with the coordinate signals. The detection circuitry typically provides for integrating a large number of spots onto photographic film.
The "resolution" of a scintillation camera refers to the ability of the camera faithfully to reproduce the spatial distribution of the radioactivity which is within the field of view of the device. The overall intrinsic resolution of the Anger camera detector is generally dependent on the ability of the detector to signal accurately the position coordinates of each scintillation event. In the Anger-type camera there is no way to determine the direction of the gamma ray which produced the scintillation event merely by detecting the location of the event. Instead, a collimator is placed in front of the detector to restrict to a narrow angle the direction of the gamma rays reaching the detector.
As a result of the need for a collimator to restrict the angle of the incident gamma rays, the Anger camera has a relatively low sensitivity, which is defined as that fraction of the gamma rays emanating from the source which actually reach the detector to produce an event that contributes to the image. If the collimator is less restrictive to admit more gamma rays and increase sensitivity, the spatial resolution of the camera is reduced.
Another type of camera relies on the Compton effect to determine the angle of incidence of a gamma ray. This camera does not require a collimator in order to determine gamma ray angle of incidence and a very efficient camera is, therefore, possible. The operating principle of a Compton camera is described by Martin JB, Knoll GF, Wehe DK, Dogan N, Jordanov V, Petrick N: A Ring Compton-scatter Camera For Imaging Medium Energy Gamma Rays, IEEE Trans. Nucl. Sci., 1993; 4-:972-978; and Singh M: Electronically Collimated Gamma Camera For Single Photon Emission Computed Tomography, Part I: Theoretical Considerations and Design Criteria, Med. Phys., 1983; 10(4) :421-427.
FIG. 1 shows a scatter aperture for a Compton camera which is composed of two detectors D1 and D2. Incident X- or .gamma.-ray photons interact in the first detector D1 via Compton-scattering, and the scattered photon is detected in time-coincidence in the second detector D2. Using the measurements of the energy deposited in the two detectors, and location of D1 and D2, the direction of the incident photon can be resolved to within a conical ambiguity. Alternatively, if the energy of the incident photon is known, a measurement from only one of the two detectors is needed. By collecting a large number of these interactions and subsequent data processing, the conical ambiguity can be reduced and the source intensity of incident photons recovered. The primary attraction of Compton-scatter apertures for single-photon imaging is that they potentially offer reduced counting noise or improved spatial resolution. The potential for this improvement exists because the solid-angle efficiency of a scatter-camera can be two orders of magnitude higher than a conventional mechanically-collimated system for equal spatial resolution.
Despite the large gains in raw counting efficiency, some advantage is lost in the process of recovering the source distribution by removing the intrinsic ambiguity. Further reductions in performance--especially for imaging photons having energies below 200 keV--can arise from detectors having modest energy resolution. In a Compton-scatter camera, poor energy resolution translates directly to poor spatial resolution (and large noise amplification if this blurring is unfolded).
In mechanical collimated systems, efficiency must always be traded off against spatial resolution. High resolution necessarily means poor sensitivity. This is not the case for a Compton camera. Any method of reducing the uncertainty in the direction of the scattering angle that does not reduce efficiency will improve resolution. Using detectors having better energy resolution is one way to accomplish this. "Conventional" Compton cameras employ one detector having good energy resolution and another with only modest resolution. Measurements from both detectors are typically used to determine the incident energy E.sub.0. Once the incident energy has been estimated, the measurement from the detector having the best energy resolution (E.sub.1 or E.sub.2) is used to estimate the scattering angle.